/*! \page Structure_factorDoc static structure factor

Keyword: SK

\section description Description

Electronic static structure factor given by
\f[
S({\bf k})=\frac1N\,\langle\Psi|\hat\rho_{\bf k}
\hat\rho_{-\bf k}|\Psi\rangle
\f]
where \f$ \hat\rho_{\mathbf{k}} \f$ is a Fourier component of the
electron density. The quantity is evaluated either on a full 3D grid
or on a 1D grid along a fixed direction, see keyword DIRECTION.

The structure factor \f$ S({\bf k}) \f$ can be used to
evaluate finite-size corrections to the Ewald energy, see Chiesa et al. 
<a href="http://dx.doi.org/10.1103/PhysRevLett.97.076404">Phys.
Rev. Lett. <b>97</b>, 076404 (2006)</a>. Note that \f$ S({\bf k}) \f$
as calculated contains besides the continuous electronic
component also sharp Bragg peaks, since electronic wave function
follows crystal symmetries. This also means that one cannot extract
the electronic component from a simulation performed in the primitive
cell, because all allowed k-values overlap with the Bragg peaks in such a
case.

\section options Options

\subsection reqopt Required 

None.

\subsection optopt Optional

<table>
<tr><th>Option</th><th>Type</th><th>Default</th><th>Description</th></tr>

<tr><td>NGRID</td><td>Integer</td><td>5</td>
<td>Number of grid points in one direction. Structure factor is
evaluated on NGRID points if DIRECTION is given and on NGRID^3 points
otherwise.
</td></tr>

<tr><td>GVEC</td><td>Section</td><td>reciprocal lattice vectors</td>
<td>Basis vectors \f$ {\bf g}_i \f$ defining the grid. The
section contains a 3x3 matrix of floats.</td>

<tr><td>DIRECTION</td><td>Integer</td><td>not set</td>
<td>Selects one of the three basis vectors \f$ {\bf g}_i \f$, along
which \f$ S({\bf k}) \f$ is evaluated.
</td></tr>
</table>


*/